Interim Pension and Other Postretirement Benefit Plan Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Defined Benefit Plan Disclosure [Line Items]
Contribution to postretirement medical plans			$ 8,700
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4,613	$ 4,652	9,225	$ 9,305
Interest cost	5,028	5,843	10,056	11,686
Expected return on assets	(6,978)	(7,079)	(13,956)	(14,158)
Amortization of prior service cost (credit)	(58)	(58)	(116)	(116)
Amortization of actuarial (gain) loss	3,171	3,242	6,343	6,483
Net periodic pension cost	5,776	6,600	11,552	13,200
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	4,306	3,366	8,612	6,733
Interest cost	2,660	2,653	5,320	5,306
Expected return on assets	(2,614)	(2,625)	(5,228)	(5,249)
Amortization of prior service cost (credit)	43	43	86	87
Amortization of actuarial (gain) loss	0	(334)	0	(669)
Net periodic pension cost	4,395	$ 3,103	8,790	$ 6,208
Minimum | Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected future contributions	15,000		15,000
Maximum | Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected future contributions	$ 25,000		$ 25,000